1. Field of the Invention
The present invention relates to the formation of a communication group comprising a control station and mobile station. The present invention more particularly relates to the formation of a communication group when a control station of the communication group is replaced with a new one and there is a mobile station incapable of communicating with the new control station.
2. Description of the Related Art
In some wireless communication networks, one wireless cell comprises a control station and mobile station. For example, a communication network currently being standardized by IEEE802.15, such as IEEE802.15.3a (a so-called UWB (Ultra Wide Band)), comprises a control station and mobile station (patent reference 1). A network (i.e., a communication group) controlled by one control station is called a piconet. Also, a range within which a sync signal from a control station controlling the piconet can reach is called a communication range. The inside of the communication range is called a region within the communication range of the control station. The outside of the communication range is called a region outside the communication range of the control station. The sync signal is sometimes called a management signal for network management. In a network like this, a control station (also called a parent station, master, or primary station) broadcasts to a mobile station (also called a slave or secondary station) a signal for establishing synchronization or a signal for notifying the mobile station that it belongs to the network under the control of the control station. The signal transmitted from a control station to a mobile station in order to construct a wireless network will be referred to as a sync signal in the present invention. Also, a network terminal complying with the IEEE802.15.3a standards is normally a multi-functional terminal having a mobile station function and control station function. Therefore, a mobile station can also function as a control station. However, a mobile station having no control station function can also exist.
A terminal having received a sync signal from a control station operates as a mobile station, i.e., performs predetermined processing corresponding to a communication scheme and transmits a response to the control station. This establishes synchronization between the mobile station and control station, so they can communicate with each other by exchanging necessary information such as addresses. This procedure is performed between the control station and each mobile station having received by a sync signal. Consequently, the control station having transmitted one sync signal (or a series of sync signals) and the mobile stations having responded to the sync signal form a piconet. That is, the mobile stations are members forming the piconet. The control station maintains the piconet by periodically transmitting the sync signal.
The control station of the piconet thus constructed sometimes transfers the control to a new control station. According to the IEEE802.15 MAC standards, if there is a mobile station having capability as a control station higher than that of a present control station, the present control station transfers the control to this mobile station. In this case, the new control station (called an alternative control station) may exist in a position different from the original control station, and this may produce a mobile station which exists in a region outside the communication range of the new control station. If this mobile station in the region outside the communication range is a multi-functional terminal having a control station function, it can operate as another control station and form a new piconet. [Patent reference 1] U.S. patent application Publication No. 2003169697 (Japanese Patent Laid-Open No. 2003-143644)
Unfortunately, a certain terminal functioning as a mobile station cannot operate as a control station (a mobile station like this will be referred to as a restricted mobile station hereinafter). Whether a terminal can operate as a control station is determined by, e.g., the capacity of a built-in memory and whether the terminal is driven by a battery. For example, a terminal which is driven by a battery or has a small memory capacity and hence is unable to function as a control station is designed and fabricated to exclusively function as a mobile station. A restricted mobile station like this cannot communicate with other terminals unless there is a control station including this mobile station in a region within the communication range of the control station. Accordingly, a restricted mobile station in a region outside the communication range of the alternative control station loses the means for communicating with other terminals.
Also, even when a terminal in a region outside the communication range of the alternative control station is a multi-functional terminal, a new piconet constructed by this terminal functioning as a control station does not necessarily include terminals which belong to the original piconet. If the new piconet does not include the terminals which belong to the original piconet, it may become meaningless to construct the new piconet.